This may seem superfluous, but having an additional map layer could expedite a lot of things in terms of creating maps. There are times when having a base underlayer map that is not affected by the current map layer would be super helpful. For instance, you could have a town map drawn out, then use the next map layer to put in objects (that would otherwise not be considered tokens but act as additional decoration) or roofs for buildings.
As a person who uses various art programs for various things, I *know* that the more layers you have access to, the better. Obviously for an application like roll20, having a ton of layers is cumbersome and unrealistic, but having two discrete and separate map layers would be an invaluable QoL improvement for many people. You can have whole map segments hidden under others using this function, and, depending on where the map layer is located, it can have additional useful functions (say, for instance, having this map layer placed ABOVE the token layer, creating things like bridges, hidden alcoves, etc.)
There are a few other ideas floating about that have some similar functionality (being able to pin map tiles in place, making them immovable, would do some of what this feature could do, for instance), but I think having just one more map layer would solve several problems.